The present invention is related to a solid electrolytic capacitor with high humidity resistance. More specifically, the present invention is related to a solid electrolytic capacitor with reduced leakage current under high humidity and high temperature test conditions. The present invention is also related to a method for producing the solid electrolytic capacitor.
A solid electrolytic capacitor typically comprises a porous pressed pellet of sintered powder as the anode with a wire inserted into the powder or attached to the pressed powder. An oxide layer is formed on the surface of the pellet as the dielectric. A conductor, typically an electrically conductive polymer, functions as the cathode wherein the conductor impregnates the pores of the pellet such that dielectric is between the polymer and anode. External connections, such as carbon paint, silver paint, etc., allow a negative lead, to be placed in electrical contact with the conductive polymer such as by a conductive adhesive. The wire is welded to an anode termination as the positive lead of the capacitor. The construction of a solid electrolytic capacitor with electrically conductive polymer is well known to those of skill in the art and further discussion of the general structure is not necessary.
The electrically conductive polymer, typically selected from polyaniline, polypyrrole, polythiophene, and their derivatives, provides the advantage of low equivalent series resistance (ESR) and, unlike manganese dioxide, the failure mode does not include burning or ignition. The conductive polymer layer is typically applied by either an in situ process, wherein monomer is chemically or electrochemical polymerized, or by a coating process wherein pre-formed conductive polymer is applied in the form of a dispersion.
In the process of manufacturing solid electrolytic capacitors, anodized anodes are processed through multiple dipping cycles wherein conductive polymer, and other components, is deposited onto the dielectric surface via either the in-situ process or the dispersion process. Typically, a hydrophobic insulated coating material is coated on the wire to electrically isolate the positive anode and negative cathode of the solid capacitor. The hydrophobic characteristic of the coating material inhibits chemical solution from climbing, or wicking, up the wire. It is preferable to remove portions of the coating material, and any oxide on the surface of the wire, in the region of anode termination attachment to insure an adequate solder bond. The coating material and dielectric layer is typically removed by a mechanical method or a laser method to expose the metal of the wire thereby enhancing the welding strength between the wire and anode termination. The distance between the conductive polymer, as the negative electrode of the capacitor, and the exposed metal portion of the wire, as the positive electrode of the capacitor, is unfortunately limited by manufacturing limitations. Under harsh application conditions, such as high temperature and high relative humidity, there could be by-pass conduction between the conductive polymer and the exposed tantalum metal leading to increases in leakage current and, in extreme conditions, a direct electrical short. By-pass conduction as used herein refers to a conduction path around the functional dielectric.
To mitigate these issues the humidity resistance of solid electrolytic capacitors has been improved by different approaches in the art. U.S. Pat. Nos. 6,556,427 and 5,938,797, for example, describe different methods of manufacturing a solid electrolytic capacitor by improving the adhesion property between the solid electrolyte and the carbon or graphite paint layers applied thereon. U.S. Pat. No. 8,848,342 describes a method for overcoming the problem of conductive polymer layer delamination by carefully controlling the conductive polymer coating configuration and manner of deposition. U.S. Pat. No. 7,483,259 describes a method of overlaying a barrier layer with a three-dimensional cross-linked network on the solid electrolyte surface to improve adhesion. The improved methods of humidity resistance described above only solve the leakage current across the dielectric layer and do not mitigate the conductive path by-passing, or circumventing, the dielectric.
In spite of the advances in the art there is still a significant need to reduce, and preferably eliminate, the leakage current occurring from current by-passing, or circumventing, the dielectric. The present invention addresses this previously untreated mechanism of leakage current thereby mitigating a potential failure mode of a capacitor and increasing the reliability under harsh conditions such as high temperature and high relative humidity.